BEAUTY IN THE EYE OF HUBBLE

A dying star, IC 4406, dubbed the
"Retina Nebula" is revealed in this month's
Hubble Heritage image. Like many other so-called
planetary nebulae, IC 4406 exhibits a high degree
of symmetry; the left and right halves of the Hubble
image are nearly mirror images of the other. If
we could fly around IC4406 in a starship, we would
see that the gas and dust form a vast donut of material
streaming outward from the dying star. From Earth,
we are viewing the donut from the side. This side
view allows us to see the intricate tendrils of
dust that have been compared to the eye's retina.
In other planetary nebulae, like the Ring Nebula
(NGC 6720), we view the donut from the top.

The donut of material confines the
intense radiation coming from the remnant of the
dying star. Gas on the inside of the donut is ionized
by light from the central star and glows. Light
from oxygen atoms is rendered blue in this image;
hydrogen is shown as green, and nitrogen as red.
The range of color in the final image shows the
differences in concentration of these three gases
in the nebula.

Unseen in the Hubble image is a larger
zone of neutral gas that is not emitting visible
light, but which can be seen by radio telescopes.

One of the most interesting features
of IC 4406 is the irregular lattice of dark lanes
that criss-cross the center of the nebula. These
lanes are about 160 astronomical units wide (1 astronomical
unit is the distance between the Earth and Sun).
They are located right at the boundary between the
hot glowing gas that produces the visual light imaged
here and the neutral gas seen with radio telescopes.
We see the lanes in silhouette because they have
a density of dust and gas that is a thousand times
higher than the rest of the
nebula. The dust lanes are like a rather open mesh
veil that has been wrapped around the bright donut.

The fate of these dense knots of material
is unknown. Will they survive the nebula's expansion
and become dark denizens of the space between the
stars or simply dissipate?

This image is a composite of data
taken by Hubble's Wide Field Planetary Camera 2
in June 2001 by Bob O'Dell (Vanderbilt University)
and collaborators and in January 2002 by The Hubble
Heritage Team (STScI). Filters used to create this
color image show oxygen, hydrogen, and nitrogen
gas glowing
in this object.